Mendel also described how genes from one generation are passed onto the next generation. When an organism reproduces, one copy of each of their genes is randomly chosen for inclusion in their sex cells (egg and sperm), now termed the Law of Segregation. He also noted that each observable trait was independently passed down from other traits, the Law of Independent Assortment. For example, the smoothness of the seed shape was transmitted independently from the color of the flower. Each observable trait could be traced back to a single gene. These rules are now called Mendelian genetics.

We now know that Mendelian genetic rules are too simplistic for some traits. Some traits are coded by more than one gene. Furthermore, some alleles are co-expressed such as red flowers crossed with white flowers to produce pink flowers. This is also seen with human blood typing. A and B blood types are co-dominant alleles.